Information is recorded on and read from a moving magnetic tape with a magnetic read/write head positioned next to the tape. The magnetic "head" may be a single head or, as is common, a series of read/write head elements stacked individually and/or in pairs within the head unit. Data is recorded in tracks on the tape by moving the tape lengthwise past the head. The head elements are selectively activated by electric currents representing the information to be recorded on the tape. The information is read from the tape by moving the tape longitudinally past the head elements so that magnetic flux patterns on the tape create electric signals in the head elements. These signals represent the information stored on the tape.
Data is recorded on and read from each of the parallel tracks on the tape by positioning the head elements at different locations across the tape. That is, head elements are moved from track to track as necessary to either record or read the desired information. Movement of the magnetic head is controlled by an actuator operatively coupled to some type of servo control circuitry. Tape drive head positioning actuators often include a lead screw driven by a stepper motor, a voice coil motor, or a combination of both. The carriage that supports the head is driven by the actuator along a path perpendicular to the direction that the tape travels. The head elements are positioned as close to the center of a track as possible based upon the servo information recorded on the tape.
The head elements must be precisely positioned relative to the tape for the tape drive to operate properly. Ideally, the position of the head elements relative to the tape in the azimuth, yaw and zenith rotational directions does not change. The rotational position or "alignment" of the head elements is typically set during the manufacture of the tape drive. The head elements should move in only one direction, as described above, straight (usually up and down) across the width of the tape. Once the head elements are aligned, even small rotational movements of the head or the head elements may cause misalignment and higher incidences of read and write errors. Maintaining the pre-set alignment in the azimuth direction is particularly important for head assemblies in which a read element is positioned immediately downstream of a write element so that the read element can accurately check data as it is written. Alignment in the azimuth direction is often the most difficult to maintain because the head is usually mounted to a vertical face on the supporting carriage. This surface mount provides inherent stability in the zenith and yaw directions, but not in the aximuth directions.